The present invention relates to a method of inspecting apertured mask sheets used as shadow masks in colour display tubes.
The apertures in shadow masks are formed by etching metal sheet material such as mild steel sheet. The etching is done chemically after contact-exposure printing using a master pattern etching mask. In order to ensure the correct colour reproduction of an object at the cathodoluminescent screen which comprises triplets of phosphor elements, three electron beams converge at the screen on their respective phosphor elements to produce a multicoloured image. Defects in the shadow mask apertures may give rise to faults in the image, for example, the colour is defective due to the microspot at the screen being significantly wider than a triplet element as a result of the shadow mask aperture being too wide, or the brightness is uneven because the microspots are too small as a result of the aperture being much smaller than specified. If such defects are the result of a processing failure, then corrective action must be taken promptly to avoid too many rejects. Accordingly, it is desirable that the etched shadow mask sheets be inspected on-line. Also, it is desirable that the method of inspection be readily adaptable to the mixture of different sizes of shadow masks which may be produced. The method ought also to be adaptable to different aperture sizes, not only between one size of shadow mask and the next, but also between the centre and edges of any one mask sheet.
From U.S. Pat. No. 4,641,256, a method is known to measure the transmission of energy through the aperture pattern to verify that the shadow masks are suitable for the intended use. The transmission of inspection areas of the shadow mask is measured and compared to transmission limits. If the total transmission or the transmission of any of the inspection areas fails to fall within the transmission limits, the shadowmask can be failed. This method, however, is capable only of indicating the average size of the shadow mask apertures. Other errors, for instance oddly shaped apertures, with substantially "normal" average size, however, go undetected. As the picture quality is increased, the need for improved methods of inspecting apertured mask sheets increases. It is an object of the invention to provide a method of inspecting apertured mask sheets with improved diagnostic capabilities.